The membrane impermeability of erythrocytes which have been lysed in hypotonic solutions can be restored by incubation at 37 degrees in the presence of isotonic NaCl. The effects of the methods used to prepare the erythrocyte membranes and of the conditions employed for resealing are being explored. Especial attention is being paid to changes which occur when membranes are washed repeatedly, to the effects of ATP, chelating agents, ionic strength, magnesium and calcium ions upon the process. The effects of erythrocyte aging, in vivo and in vitro, upon the resealing process will be studied. Attempts are underway to prepare well washed ghosts which, upon resealing, will be cation impermeable.